A technique will be developed to measure the concentration of a fluorophore in vivo, from detection of the fluorescence to reflected light ratio using a specialized fiber optic bundle which limits the probed tissue volume. This technique will allow real-time in vivo optical biopsy of any fluorophores from a tissue. By limiting the probe (fiber optic) area on the tissue to smaller than the typical scattering length, the detected signal is insensitive to the tissue's natural absorption characteristics, and is proportional to only the fluorophore concentration. Then by dividing the fluorescent light by the reflected light, the variations in scattering coefficient are eliminated from the measurement. For example, the fluorescence signal from dark and light pigmented skin should be equal, for equal concentrations of fluorophore. In photodynamic therapy, it can be used to measure the uptake of photosensitizer within tissues, and may eventually replace time consuming tissue extraction methods which are currently used. A special fiber optic bundle will be created which samples a number of small spots on the tissue, and integrates the signals into a single measurement. This process retains the insensitivity to the tissue type while allowing lower light intensities to be used. PROPOSED COMMERCIAL APPLICATIONS: The fiber optic based system outlined in this proposal would be an extremely useful tool for all clinics involved in photodynamic therapy research and clinical trials. Essentially all of these labs need to quantitate the photosensitizer uptake in tissue for accurate treatment planning. Current methods are time consuming and expensive. Beyond photodynamic therapy, this device would also have potential to provide pharmacokinetic studies of fluorescent tracer drugs injected in vivo.